Piezoeletric wet etch process with reduced resist lifting and controlled undercut

ABSTRACT

A microelectronic device containing a piezoelectric thin film element is formed by oxidizing a top surface of a piezoelectric layer with an oxygen plasma, and subsequently forming an etch mask containing photoresist on the oxidized top surface. The etch mask is conditioned with an oven bake followed by a UV bake. The piezoelectric layer is etched using a three step process: a first step includes a wet etch of an aqueous solution of about 5% NH 4 F, about 1.2% HF, and about 18% HCl, maintaining a ratio of the HCl to the HF of about 15.0, which removes a majority of the piezoelectric layer. A second step includes an agitated rinse. A third step includes a short etch in the aqueous solution of NH 4 F, HF, and HCl.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under U.S.C. §119(e) ofU.S. Provisional Application 62/018,756 (Texas Instruments docket numberTI-74897PS), filed Jun. 30, 2014, which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

This invention relates to the field of microelectronic devices. Moreparticularly, this invention relates to piezoelectric layers inmicroelectronic devices.

BACKGROUND OF THE INVENTION

Some microelectronic devices containing a component with a piezoelectricelement are fabricated by forming an etch mask on the layer ofpiezoelectric material and etching the piezoelectric material to definethe piezoelectric element. Etching the piezoelectric material isproblematic. Dry etching the piezoelectric material is difficult forlayers over a micron thick due to etch residue buildup and mask erosion.Wet etching the piezoelectric material undesirably undercuts the mask inan uncontrollable manner and lifts the mask from the piezoelectricmaterial, producing undesired profiles in the piezoelectric element.

SUMMARY OF THE INVENTION

The following presents a simplified summary in order to provide a basicunderstanding of one or more aspects of the invention. This summary isnot an extensive overview of the invention, and is neither intended toidentify key or critical elements of the invention, nor to delineate thescope thereof. Rather, the primary purpose of the summary is to presentsome concepts of the invention in a simplified form as a prelude to amore detailed description that is presented later.

A microelectronic device containing a piezoelectric thin film element atleast 1 micron thick is formed by oxidizing a top surface of apiezoelectric layer with an oxygen plasma, and subsequently forming anetch mask containing photoresist on the oxidized top surface. The etchmask is conditioned with an oven bake followed by an ultraviolet (UV)bake. The piezoelectric layer is etched using a three step process: afirst step includes a wet etch of an aqueous solution of about 5% NH₄F,about 1.2% HF, and about 18% HCl, maintaining a ratio of the HCl to theHF of about 15.0, which removes a majority of the piezoelectric layer. Asecond step includes an agitated rinse. A third step includes a shortetch in the aqueous solution of NH₄F, HF, and HCl.

DESCRIPTION OF THE VIEWS OF THE DRAWING

FIG. 1A through FIG. 1P are cross sections of a microelectronic devicecontaining a piezoelectric thin film element, depicted in successivestages of an example fabrication sequence.

FIG. 2 depicts the microelectronic device of FIG. 1J in an alternateagitated rinse process.

FIG. 3 depicts the microelectronic device of FIG. 1J in another versionof the agitated rinse process.

FIG. 4 depicts the microelectronic device of FIG. 1J in a furtherversion of the agitated rinse process.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The present invention is described with reference to the attachedfigures. The figures are not drawn to scale and they are provided merelyto illustrate the invention. Several aspects of the invention aredescribed below with reference to example applications for illustration.It should be understood that numerous specific details, relationships,and methods are set forth to provide an understanding of the invention.One skilled in the relevant art, however, will readily recognize thatthe invention can be practiced without one or more of the specificdetails or with other methods. In other instances, well-known structuresor operations are not shown in detail to avoid obscuring the invention.The present invention is not limited by the illustrated ordering of actsor events, as some acts may occur in different orders and/orconcurrently with other acts or events. Furthermore, not all illustratedacts or events are required to implement a methodology in accordancewith the present invention.

FIG. 1A through FIG. 1P are cross sections of a microelectronic devicecontaining a piezoelectric thin film element, depicted in successivestages of an example fabrication sequence. Referring to FIG. 1A, themicroelectronic device 100 has a structural member 102, which may be amonolithic substrate, or may be a movable member such as a membrane orbeam. The structural member 102 may include, for example, silicon,sapphire, ceramic, glass, and/or plastic. The structural member 102 mayinclude a dielectric layer at a top surface to provide electricalisolation from the subsequently-formed piezoelectric element. A lowercontact layer 104 is formed over the structural member 102. The lowercontact layer 104 may include, for example, 75 nanometers to 200nanometers of platinum with an adhesion layer of titanium. Othermaterials in the lower contact layer 104 are within the scope of theinstant example. A layer of piezoelectric material 106 at least 1 micronthick is formed on the lower contact layer 104. The layer ofpiezoelectric material 106 may include primarily lead zirconiumtitanate. An upper contact layer 108 is formed on the layer ofpiezoelectric material 106. The upper contact layer 108 may include, forexample, 75 nanometers to 200 nanometers of platinum.

A top contact mask 110 is formed over the upper contact layer 108 so asto cover an area for a subsequently-formed upper contact. The topcontact mask 110 may include photoresist formed by a photolithographicprocess, and may optionally include an organic anti-reflection layersuch as a bottom anti-reflection coat (BARC).

Referring to FIG. 1B, a first dry etch process 112 removes the uppercontact layer 108 of FIG. 1A where exposed by the top contact mask 110to form a top contact 114. The first dry etch process 112 may be, forexample, an inductively-coupled reactive ion etch (ICP-RIE) usingchlorine radicals, oxygen radicals and argon ions as depicted in FIG.1B. An example first dry etch process 112 for a 200 millimeter waferflows boron trichloride (BCl₃) at 30 standard cubic centimeters perminute (sccm), chlorine (Cl₂) at 60 sccm to 150 sccm, oxygen (O₂) at 5sccm to 20 sccm, and argon (Ar) at 5 sccm to 30 sccm, maintaining apressure of 8 millitorr to 12 millitorr. Radio frequency (RF) power isapplied to an upper electrode over the microelectronic device 100 at 400watts to 600 watts, which is about 1.3 watts/cm² of wafer area to about1.9 watts/cm², and RF power is applied to a lower electrode under themicroelectronic device 100 at 250 watts to 350 watts, which is about 0.8watts/cm² to about 1.1 watts/cm², for 100 second to 250 seconds, untilthe upper contact layer 108 is removed where exposed by the top contactmask 110. The first dry etch process 112 may be endpointed to determinewhen the upper contact layer 108 is removed. The first dry etch process112 does not remove a significant amount of the piezoelectric layer 106.

Referring to FIG. 1C, an overetch process 116 of the first dry etchprocess 112 of FIG. 1B conditions a top surface 120 of the layer ofpiezoelectric material 106 where exposed by the top contact 114. Theoveretch process 116 includes chlorine radicals and oxygen radicals, andis free of argon. Work performed in pursuit of the instant example hasshown that argon in the overetch process 116 deleteriously affectsadhesion of a subsequently-formed piezoelectric element mask. In oneversion of the instant example, the overetch process 116 may terminatethe argon flow and reduce the RF power to the lower electrode by 50watts, which is about 0.16 watts/cm², while maintaining other parametersthe same as in the first dry etch process 112. The overetch process 116may be a timed process, for example 90 seconds to 150 seconds long. Theoveretch process 116 may erode at least a portion of the top contactmask 110.

Referring to FIG. 1D, oxygen radicals from an oxygen plasma process 118oxidize the top surface 120 of the layer of piezoelectric material 106where exposed by the top contact 114 and remove the remaining topcontact mask 110. The oxygen plasma process 118 may be performed in asame chamber as the first dry etch process 112 of FIG. 1B and theoveretch process 116 of FIG. 1C, which may advantageously reducefabrication cost and complexity of the microelectronic device 100. Anexample oxygen plasma process 118 flows O₂ at 100 sccm to 250 sccm whilemaintaining a pressure of 18 millitorr to 30 millitorr. RF power isapplied to the upper electrode at 400 watts to 700 watts, which is about1.3 watts/cm² to about 2.2 watts/cm², and RF power is applied to thelower electrode at 200 watts to 250 watts, which is about 0.6 watts/cm²to about 0.8 watts/cm². The oxygen plasma process 118 may be endpointedso as to terminate when the top contact mask 110 is removed, or may be atimed etch, for example 75 seconds to 125 seconds. Oxidation of the topsurface 120 of the layer of piezoelectric material 106 by the oxygenplasma process 118 may advantageously increase adhesion of thesubsequently-formed piezoelectric element mask. In one version of theinstant example, the top contact mask 110 may be removed without using awet process, such as a wet etch or a wet clean. Removing the top contactmask 110 without using a wet process may advantageously preserve theoxidation of the top surface 120 of the layer of piezoelectric material106.

Referring to FIG. 1E, an oxygen over-ash process 122 further oxidizesthe top surface 120 of the layer of piezoelectric material 106 whereexposed by the top contact 114. In one version of the instant example,the oxygen over-ash process 122 may maintain the parameters of theoxygen plasma process 118 of FIG. 1D for a fixed time, for example 50seconds to 100 seconds. Oxidation of the top surface 120 of the layer ofpiezoelectric material 106 by the oxygen over-ash process 122 mayadvantageously further increase adhesion of a subsequently-formedpiezoelectric element mask.

Referring to FIG. 1F, a piezoelectric element mask 124 is formed overthe layer of piezoelectric material 106 so as to cover an area for thesubsequently-formed piezoelectric element. In the instant example, thepiezoelectric element mask 124 covers and extends past the top contact114 by at least a micron. The piezoelectric element mask 124 may includephotoresist, for example a negative photoresist comprisingpolyisoprene-based polymer, formed by a photolithographic process, andmay optionally include a BARC layer. A solvent such as an aqueous aminesolution with a pH of 11 to 12 may optionally be applied to themicroelectronic device 100 prior to forming the piezoelectric elementmask 124 to promote adhesion of the photoresist to the top surface 120.

Referring to FIG. 1G, the microelectronic device 100 is placed on afirst heated structure 126 such as a hot plate or oven plate. Themicroelectronic device 100 is baked at 180° C. to 190° C. for 45 minutesto 90 minutes. The microelectronic device 100 may be baked in anatmospheric ambient, or may be baked in an inert ambient such as anitrogen ambient. Baking the microelectronic device 100 mayadvantageously improve adhesion of the piezoelectric element mask 124 tothe layer of piezoelectric material 106 and improve resistance of thepiezoelectric element mask 124 to a subsequent wet etch process.

Referring to FIG. 1H, the microelectronic device 100 is placed on asecond heated structure 128 and exposed to UV radiation 130 from one ormore UV sources 132 for 90 seconds to 150 seconds. The UV sources 132may be, for example, low intensity UV sources with power densities from80 milliwatts/cm² to 108 milliwatts/cm² or high intensity UV sourceswith power densities from 225 milliwatts/cm² to 280 milliwatts/cm². Themicroelectronic device 100 may be heated to 200° C. to 250° C. whileexposed to the UV sources 132. Baking the piezoelectric element mask 124under the UV sources 132 may advantageously further improve adhesion ofthe piezoelectric element mask 124 to the layer of piezoelectricmaterial 106 and further improve resistance of the piezoelectric elementmask 124 to the subsequent wet etch process.

Referring to FIG. 1I, a first wet etch 134 of a three step processremoves the layer of piezoelectric material 106 of FIG. 1H where exposedby the piezoelectric element mask 124, to form a piezoelectric element136. The first wet etch 134 is an aqueous solution of 4.5 percent to 5.5percent ammonium fluoride (NH₄F), 1.1 percent to 1.3 percenthydrofluoric acid (HF), and 16.5 percent to 19.5 percent hydrochloricacid (HCl), wherein a ratio of the HCl to the HF is maintained at avalue of 14.5 to 15.5. The first wet etch 134 may expose the lowercontact layer 104. Maintaining the ratio of the HCl to the HFadvantageously controls horizontal etching of the layer of piezoelectricmaterial 106 so that an undercut distance 138 of the piezoelectricelement 136 under the piezoelectric element mask 124 is less than threetimes a thickness 140 of the layer of piezoelectric material 106.Oxidizing the top surface 120 of the layer of piezoelectric material 106as described in reference to FIG. 1D and forming the piezoelectricelement mask 124 as described in reference to FIG. 1E through FIG. 1Hadvantageously reduce separation of the piezoelectric element mask 124from the top surface 120 during the first wet etch 134. Etch residue 142which has a low solubility in water may be present on the lower contactlayer 104 after the first wet etch 134 is completed.

Referring to FIG. 1J, the microelectronic device 100 is exposed to anagitated rinse process 144, which is a second step of the three stepprocess. The agitated rinse process 144 may include substantially alldeionized water (DI H₂O), with a source of mechanical agitation. In theinstant example, the source of mechanical agitation may be nitrogenbubbling, indicated in FIG. 1J as bubbles 146. Other methods ofproviding the mechanical agitation are within the scope of the instantexample. The mechanical agitation of the agitated rinse process 144 mayadvantageously remove a portion of the etch residue 142 and mayadvantageously remove a crust on remaining etch residue 142 tofacilitate removal in a third step of the three step process.

Referring to FIG. 1K, a second wet etch 148, with an aqueous solutionhaving substantially a same composition as the first wet etch 134 ofFIG. 1I, removes any remaining etch residue 142 of FIG. 1J. The secondwet etch 148 may be performed for a short time, for example 5 seconds to20 seconds. The undercut distance 138 of the piezoelectric element 136under the piezoelectric element mask 124 remains less than three timesthe thickness 140 of the piezoelectric element 136. After the second wetetch 148 is completed the aqueous solution of the second wet etch 148 isremoved by a rinse process, for example a series of three rinse and dumpsteps.

Referring to FIG. 1L, oxygen radicals 150 of an oxygen ash processremove the piezoelectric element mask 124. The oxygen radicals 150 donot remove a significant amount of the piezoelectric element 136. A wetclean is not used to remove the piezoelectric element mask 124, nor is awet clean used to remove residue following the oxygen ash process.

Referring to FIG. 1M, a bottom contact mask 152 is formed over the topcontact 114 and the piezoelectric element 136, extending partway ontothe lower contact layer 104. The bottom contact mask 152 may includephotoresist, and may be thicker than the top contact mask 110 of FIG. 1Aso as to cover the piezoelectric element 136. The bottom contact mask152 may extend past the piezoelectric element 136 by at least 1 micron.

Referring to FIG. 1N, a second dry etch process 154 removes the lowercontact layer 104 of FIG. 1M where exposed by the bottom contact mask152 to form a bottom contact 156. The second dry etch process 154 may besimilar to the first dry etch process 112 described in reference to FIG.1B.

Referring to FIG. 10, oxygen radicals 158 from an oxygen plasma processremove the bottom contact mask 152. A wet clean is not used to removethe bottom contact mask 152, nor is a wet clean used to remove residuefollowing the oxygen ash process.

FIG. 1P shows the completed piezoelectric element 136. Forming themicroelectronic device 100 according to the process described herein mayprovide the piezoelectric element 136 with a desired extension past thetop contact 114 and a desired side profile 160 which is substantiallyvertical.

FIG. 2 depicts the microelectronic device of FIG. 1J in an alternateagitated rinse process. In the instant example, the agitated rinseprocess 144 is a DI H₂O rinse with ultrasonic power applied to thedeionized water, possibly a megasonic process, generating ultrasonicwaves 162 in the deionized water which provide the agitation. Theultrasonic waves 162 may be advantageously effective at eliminating theetch residue 142 and may be advantageously effective at removing thecrust on remaining etch residue 142 to facilitate removal in the secondwet etch process 148 of FIG. 1K.

FIG. 3 depicts the microelectronic device of FIG. 1J in another versionof the agitated rinse process. In the instant example, the agitatedrinse process 144 is a DI H₂O rinse which is stirred by a stirringmechanism 164, generating flow in the deionized water which provides theagitation. The agitation may be advantageously effective at eliminatingthe etch residue 142 and may be advantageously effective at removing thecrust on remaining etch residue 142 to facilitate removal in the secondwet etch process 148 of FIG. 1K.

FIG. 4 depicts the microelectronic device of FIG. 1J in a furtherversion of the agitated rinse process. In the instant example, theagitated rinse process 144 is a DI H₂O spray rinse 166, possibly a highpressure spray rinse, for example above 100 pounds per square inch(psi), which is provided by one or more spray nozzles 168. Mechanicalforce is generated when the sprayed deionized water impacts the etchresidue 142, thus providing the agitation. The agitation may beadvantageously effective at eliminating the etch residue 142 and may beadvantageously effective at removing the crust on remaining etch residue142 to facilitate removal in the second wet etch process 148 of FIG. 1K.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only and not limitation. Numerous changes to the disclosedembodiments can be made in accordance with the disclosure herein withoutdeparting from the spirit or scope of the invention. Thus, the breadthand scope of the present invention should not be limited by any of theabove described embodiments. Rather, the scope of the invention shouldbe defined in accordance with the following claims and theirequivalents.

What is claimed is:
 1. A method of forming a microelectronic device,comprising the steps: providing a structural member; forming a lowercontact layer over the structural member; forming a layer ofpiezoelectric material on the lower contact layer; forming a top contacton the layer of piezoelectric material; exposing a top surface of thelayer of piezoelectric material, where exposed by the top contact, to anoxygen plasma; forming a piezoelectric element mask on the layer ofpiezoelectric material, extending past the top contact; thermally bakingthe piezoelectric element mask; baking the piezoelectric element maskwhile exposing the piezoelectric element mask to ultraviolet (UV)radiation; removing at least a portion of the layer of piezoelectricmaterial, where exposed by the piezoelectric element mask, by a firstwet etch comprising an aqueous solution of ammonium fluoride (NH₄F),hydrofluoric acid (HF), and hydrochloric acid (HCl); exposing themicroelectronic device to an agitated rinse process; exposing the layerof piezoelectric material to a second wet etch comprising an aqueoussolution of NH₄F, HF, and HCl; removing the aqueous solution of thesecond wet etch by a rinse process; and removing the piezoelectricelement mask.
 2. The method of claim 1, wherein the layer ofpiezoelectric material comprises primarily lead zirconium titanate. 3.The method of claim 1, wherein removing the upper contact layercomprises forming a plasma with chlorine-containing gas, oxygen andargon over the top surface.
 4. The method of claim 1, comprising anoveretch process after removing the upper contact layer and beforeexposing the top surface of the layer of piezoelectric material to theoxygen plasma, wherein the overetch process comprises forming a plasmawith chlorine-containing gas and oxygen over the top surface, the plasmaof the overetch process being substantially free of argon.
 5. The methodof claim 1, comprising an over-ash process after exposing the topsurface of the layer of piezoelectric material to the oxygen plasma andbefore forming the piezoelectric element mask, wherein the over-ashprocess exposes the top surface of the layer of piezoelectric materialto the oxygen plasma after the top contact mask is removed for 50seconds to 100 seconds.
 6. The method of claim 1, wherein thepiezoelectric element mask comprises negative photoresist.
 7. The methodof claim 1, wherein thermally baking the piezoelectric element maskcomprises baking at 180° C. to 190° C. for 45 minutes to 90 minutes. 8.The method of claim 1, wherein baking the piezoelectric element maskwhile exposing the piezoelectric element mask to UV radiation comprisesexposing the piezoelectric element mask to at least one of a lowintensity UV source with power densities from 80 milliwatts/cm² to 108milliwatts/cm² and a high intensity UV source with power densities from225 milliwatts/cm² to 280 milliwatts/cm² while heating themicroelectronic device to 200° C. to 250° C., for 90 seconds to 150seconds.
 9. The method of claim 1, wherein: the first wet etch comprisesan aqueous solution of 4.5 percent to 5.5 percent NH₄F, 1.1 percent to1.3 percent HF, and 16.5 percent to 19.5 percent HCl, wherein a ratio ofthe HCl to the HF is maintained at a value of 14.5 to 15.5; and thesecond wet etch comprises an aqueous solution of 4.5 percent to 5.5percent NH₄F, 1.1 percent to 1.3 percent HF, and 16.5 percent to 19.5percent HCl, wherein a ratio of the HCl to the HF is maintained at avalue of 14.5 to 15.5.
 10. The method of claim 1, wherein the agitatedrinse process comprises a deionized water (DI H₂O) rinse with nitrogenbubbling.
 11. The method of claim 1, wherein the agitated rinse processcomprises a deionized water (DI H₂O) rinse with ultrasonic power appliedto the DI H₂O.
 12. The method of claim 1, wherein the agitated rinseprocess comprises a deionized water (DI H₂O) rinse which is stirred by astifling mechanism.
 13. The method of claim 1, wherein the agitatedrinse process comprises a DI H₂O spray rinse.
 14. The method of claim 1,wherein removing the piezoelectric element mask is performed with an ashprocess.
 15. The method of claim 1, wherein the top contact layercomprises a layer of platinum.
 16. The method of claim 1, wherein thebottom contact layer comprises a layer of platinum over an adhesionlayer.
 17. The method of claim 1, comprising: forming a bottom contactmask over the top contact and the piezoelectric element, extendingpartway onto the lower contact layer; removing the lower contact layer,where exposed by the bottom contact mask, to form a bottom contact; andremoving the bottom contact mask.
 18. The method of claim 1, wherein thestep of removing the top contact mask is free of a wet clean.
 19. Amethod of forming a microelectronic device, comprising the steps:providing a structural member; forming a lower contact layer comprisingplatinum over the structural member; forming a layer of piezoelectricmaterial comprising primarily lead zirconium titanate on the lowercontact layer; forming an upper contact layer comprising on the layer ofpiezoelectric material; forming a top contact mask over the uppercontact layer; removing the upper contact layer, where exposed by thetop contact mask, with a plasma etch process using chlorine radicals,oxygen radicals and argon, to form a top contact on the layer ofpiezoelectric material; exposing a top surface of the layer ofpiezoelectric material, where exposed by the top contact, to an overetchprocess comprising a plasma process using chlorine radicals and oxygenradicals, the plasma process being substantially free of argon; exposingthe top surface of the layer of piezoelectric material, where exposed bythe top contact, to an oxygen plasma, the oxygen plasma removing the topcontact mask; exposing the top surface of the layer of piezoelectricmaterial, where exposed by the top contact, to an oxygen plasma over-ashprocess for 50 seconds to 100 seconds; forming a piezoelectric elementmask on the layer of piezoelectric material, extending past the topcontact; thermally baking the piezoelectric element mask at 180° C. to190° C. for 45 minutes to 90 minutes; baking the piezoelectric elementmask at 200° C. to 250° C. while exposing the piezoelectric element maskto UV radiation for 90 seconds to 150 seconds; removing at least aportion of the layer of piezoelectric material, where exposed by thepiezoelectric element mask, by a first wet etch comprising an aqueoussolution of 4.5 percent to 5.5 percent NH₄F, 1.1 percent to 1.3 percentHF, and 16.5 percent to 19.5 percent HCl, wherein a ratio of the HCl tothe HF is maintained at a value of 14.5 to 15.5; removing the aqueoussolution of the first wet etch by an agitated rinse process; exposingthe layer of piezoelectric material to a second wet etch comprising anaqueous solution of 4.5 percent to 5.5 percent NH₄F, 1.1 percent to 1.3percent HF, and 16.5 percent to 19.5 percent HCl, wherein a ratio of theHCl to the HF is maintained at a value of 14.5 to 15.5; removing theaqueous solution of the second wet etch by a rinse process; removing thepiezoelectric element mask by an ash process; forming a bottom contactmask over the top contact and the piezoelectric element, extendingpartway onto the lower contact layer; removing the lower contact layer,where exposed by the bottom contact mask, to form a bottom contact; andremoving the bottom contact mask.
 20. A method of forming amicroelectronic device, comprising the steps: providing a structuralmember; forming a layer of piezoelectric material above the structuralmember; forming a top contact on the layer of piezoelectric material;exposing a top surface of the layer of piezoelectric material, whereexposed by the top contact, to an oxygen plasma; forming a piezoelectricelement mask on the layer of piezoelectric material, extending past thetop contact; thermally baking the piezoelectric element mask; baking thepiezoelectric element mask while exposing the piezoelectric element maskto UV radiation; removing at least a portion of the layer ofpiezoelectric material, where exposed by the piezoelectric element mask,by a first wet etch comprising an aqueous solution of 4.5 percent to 5.5percent NH₄F, 1.1 percent to 1.3 percent HF, and 16.5 percent to 19.5percent HCl, wherein a ratio of the HCl to the HF is maintained at avalue of 14.5 to 15.5; removing the aqueous solution of the first wetetch by an agitated rinse process; exposing the layer of piezoelectricmaterial to a second wet etch comprising an aqueous solution of 4.5percent to 5.5 percent NH₄F, 1.1 percent to 1.3 percent HF, and 16.5percent to 19.5 percent HCl, wherein a ratio of the HCl to the HF ismaintained at a value of 14.5 to 15.5; removing the aqueous solution ofthe second wet etch by a rinse process; and removing the piezoelectricelement mask.